In the field of cranes, i.e. machines that lift loads suspended from a hook block attached to a wire rope trained over a sheave on the tip section of a lufting boom and wound on the drum of a winch, contact between the hook block and the tip section of the crane's boom is undesirable because of the possibility of damage to the crane as a consequence. Reeling in the wire rope on the drum will, of course, cause the hook block to approach the boom tip. So also will lowering tne boom, i.e. increasing the angle between the boom and a vertical line through the boom foot pivot axis, because of the geometry involved. The center of the arc traced by a point on the boom tip is the boom foot pivot axis, but the comparable center for the wire rope where it contacts the sheave on the boom tip is the rope's point of contact with either that drum or sheave which is fixed relative to the crane's upper structure. These two centers cause their respective arcs to diverge as the boom is lowered; the amount of such divergence increasing as the distance between the two centers increase. This divergence is compensated for by the hook block moving toward and away from the boom tip as the boom is respectively lowered and raised.
To prevent contact between the hook block and the tip section, automatic limiting devices, which are commonly called anti-two block devices, have been utilized in the prior art. These devices preclude lowering of the boom and/or further reeling in of the hoist line when the hook block attached thereto is a pre-determined distance from the tip section. Such prior art devices have utilized a mechanism suspended from the boom tip which trips a limit switch, when the hook block contacts that mechanism, to break an electrical circuit and thereby deactivate solenoids which must be activated for operation of the hook block hoist raise and boom hoist lower controls. Such arrangements are undesirable because the switch and the associated tripping mechanism are exposed to the ravages of the elements, are difficult to install and service due to the boom tip location of these components, especially on a tower crane where the boom cannot be readily lowered to the ground, and are subjected to possible damage during erection or disassembly of the crane and during operation of the crane by hoisting the hook block at high speed into the tripping mechanism. In addition, raising the hook block while simultaneously swinging the crane often causes the tripping mechanism to contact the hoist line thereby creating a tendancy for the tripping mechanism itself to "climb" the wire rope and strike the boom tip, resulting in the possibility of damage to the crane. Finally, placement of any weight on the boom not absolutely necessary for the actual support and lifting of a load, especially weight concentrated at the tip section, diminishes the lifting capacity of the crane.
The present invention provides an anti-two block system which automatically precludes contact between the hook block and boom tip section and which is devoid of the aforementioned problems, disadvantages and limitations of the prior art anti-two block devices.